Traveling wave tubes



Dec. 22, 1959 D. c. ROGERS 2,918,593

TRAVELING WAVE TUBES Filed Aug. 24, 1955 I 2 Sheets-Sheet 1" Inventor D. C. ROGERS Attorney Dec. 22, 1959 D. (3. ROGERS 2,918,593

TRAVELING WAVE TUBES Filed Aug. 24, 1955 2 Sheets-Shet 2 Inventor D. C. ROGERS By M;

Attorney United States Patent ice priorityyappliciition Great-Britain September '21, 1954 "1' Claim. -(Gl. 31 3-84) The-pre'sent-invention relates to apparatus for providin'g a magneticifieldnalong a given axis byfmeans of a id continuous magneti structure and is "particularly but notufexclusively concerned with such .apparatusior use with traveling?- wave tubes.

ulnttra'veling wave apparatus an I electron beam is pro- 'ected fromrant electron 'gun along a given axis to :interact with' the electromagnetie'field of some form ofslow wave -structure, typically'a'helix. "A traveling wave oscillator rr'equires rat: leastian output connection for the slow wave tf'str'ucture'and a traveling wave amplifier requires both ninputrand: output connections. These-connections com- .t prise eitheri coaxial line or hollow waveguide transmis- -:sion paths external to the traveling wave tubeand con aresponding-transformer or: pick-up arrangements as'socistated with the :-slow wave' structure, usually within the -tienve lope ofthe traveling wave tube. Although the problemwithwhich the present invention is concerned is more -acute whenrectangular waveguides are used, the invenution; nevertheless is also applicable when a'coaxial line deed for the slow wave structure, transverse to the -longitudinal axis thereof, is usedg-in-the presentspecification and claim,r th'erefore, the expression waveguide Min-the-appropriate context is to be taken to include -'a --coaX-ialtransmission line.

Thelong electron beams used in traveling wave tubes demand the use of magnetic focussing fields which are st-raight-and-uniform in intensity over distances-of-some 25:to-SO centimetres. Such-magnetic fields can readily -be ,-produced by uniformallywound' 'solenoids, but a -dift-ficulty arises in practicedue to the'need to accommodate the waveguideinput and/or output feeds for the travel ring --wave tube. These waveguides interrupt the con- -tinuityof the winding'and cause an irregularity in the -smagnet-ic-field. To some extent-this can-be prevented by sthe use of solenoids-with. a meandiameter large incom- --parison with the axial interruption of the winding, but -it-isw inconvenient touse amean-diameter more than four .-orfivetimes greater than the length of the interruption, and,-while this reduces the magnitude of the irregularity, itedoes .--not-climinate -it. Furthermore large diameter "windings result. in a considerable waste of 'energizing .,power,-for thewinding resistance is higherthan would otherwise be necessary.

It is tin-object of the present invention to provide means for producing the magnetic field required for focussing' the electron beam of a travelingwave tube -without the necessity for large diameter solenoids, with onseque'ntreduction-in'the 'size and weight of the appara'tus'rand economy inpower consumption.

This is achieved in that the regions betweenand beyond the input and output waveguides, in the case where two waveguides are required, are energised by winding whose internal diameter is small (restricted only by 'theneed to'accommodate the tube), encased in screens ofma'gnetic material, and the gaps between them'are energised by a second series of solenoids wound on Patented Dec. 22, 1959 of an electromagnet.

While the invention 'has been conceived with' theproblems of traveling wavetub'es' primarily in -mind, it is 5 evident' that the invention' is J applicable to other types of apparatus where it is 'de'sired' to regulatethe magnetic field across a discontinuity between ap'a'irof solenoids. "While, usually, the"problem is 'to preserve uniformity of field conditions across the "physical discontinuity,=there are cases where special shapingl of the fluxlir'res orev'en the inullification of the field is' required.

According toone aspectof the present invention,'-therefore, there-is providedapparatus "for providing a mag netic field-along a given axis'by m'eans of 'a discon- "tinuous magnetic structure surrounding the said axis comprising a pair of 'iriner solenoidsabout the said i axiseach provided *withmagnetic screening fm'eans, "with gap between the solenoids and the two' screeningmeans,

and amouter solenoid. placed'to one side of thesaid gap 20 of apparatus-which iincludes means for mountingand aligning the traveling wave tube and part ofthe' ancillary waveguide feeds.

According toa further" aspect--of the invention, there is provided traveling wave apparatus 'eomprising, asa unitary assembly,'-a pair-of coaxial innersolenoids mounted end to-- end means for-mounting a-traveling-wave tube along the-axis of the said -solenoids, a waveguide 'for coupling to the said traveling wave tube mount-ed between the two said solenoids, magnetic screens closely surrounding the respective inner solenoids and an outer solenoid adjacent to the said waveguide surrounding "a portion of one of said magnetic screens and arranged so as to compensate for the interruption of :themagnetic path between the two inner solenoids.

While ithasbeen stated .above that,-for a traveling wave tube, a uniform magnetic-field is' required, it--has been found in traveling wave amplifiers that though, with a uniformfield, an unmodulated electron beam can be projected through the helix or alternative: slow Wave structure with considerably less than one percent 'ofthe 'beam 'current'being intercepted-under conditions of maximum signal output the bunching of the beamresults in an increase in mean beam diameter and a consequent increase of beam current interception towards the-output end 'of the helix. This can be overcome by increasing the strength of the 'beam confining magneticfield towards the end of the helix, or alternative slow wave structure, and embodiments of the present invention-can readily be adapted. to provide this increase in magnetic Embodiments ofthe invention will now be described with reference to the accompanying drawings in which:

Figs. 1 to 3 show various modificationsof magnetic field producing'apparatus according to .the invention for use with traveling wave tubes; and

Fig. 4shows a traveling wave tube inserted in a-unitary' assembly'of focussing coils, waveguide'feedersand tube-mounting means, the apparatus being shown in crosssection.

In Fig. 1' three inner solenoids, 1, 2 and 3, are shown mounted end to end about a common axis 4-5. The internal diameter of these solenoids is such as to. provide justsufficient space for the insertion of a traveling wave tube of uniform envelope diameter therealong. -Airgaps 6 and 7 separate solenoids 1 and--2, and 2 and respectively, and allow for the introduction of waveguide feeds.

For normal focu's'sing purposes it is required that-there be'auniformmagneticfield extendingalong the axis between the points marked 4 and 5. The regions between and beyond the airgaps 6 and 7 are energised by the solenoids 2, 1 and 3 respectively. These solenoids are encased in respective magnetic screens 8, 9 and 10, the screens extending transversely towards the common axis on either side of the airgaps as indicated at 11, 12, 13 and 14. In accordance with the present invention, the airgap regions 6 and 7 are energised by means of the three outer solenoids 15, 16 and 17 wound over the respective inner solenoids 1, 2 and 3 and their respective screens. The transverse portions 11, 12, 13 and 14 of the magnetic screens function as pole pieces, the gaps 6 and 7 being effectively in series in a magnetic circuit which is completed by end-plates 18 and 19 and an intervening external magnetic shield 20. Ideally the shield 20 would consist of a cylinder of ferromagnetic material, but, since this would not allow ready access for the waveguides, it is more convenient to use a squirrel-cage arrangement of rods of low reluctance material which also may be used as tie-rods to clamp up a complete assembly, such as that shown in Fig. 4, for mounting the traveling wave tube. By proper choice of the ampere-turns in the outer solenoids 15, 16 and 17 relative to the ampere-turns of the inner solenoids 1, 2 and 3, the average field across the gaps 6 and 7 can be made equal to the field on the axis within the inner solenoids. By making the external diameter of the shields 9, 8 and not less than about twice the length of the gaps 6 and 7, the field across these gaps can be made substantially uniform. Since the fields impinging upon the outer and the inner surfaces of transverse screen portions 11, 12, 13 and 14 are equal in magnitude and in the same direction, provided they are thin, the central apertures therein for accommodating the traveling wave tube do not disturb the field. Thus by means of the invention it is possible to establish a magnetic field substantially uniform from one end 4 of the axis, through the air gaps 6 and 7, to the other end 5, while the external diameter of the assembly need not exceed some three times the length of each of the airgaps.

The arrangement described above with reference to Fig. 1 has several variants which will be apparent to those skilled in the art. Thus, since the magnetic fields impinging on either side on each of the transverse portions 11 to 14 of the magnetic screens are equal, at least on the axis, in some embodiments of the invention they may be omitted, the screens becoming simple cylinders as illustrated in Fig. 2. In any given case it is best to determine by experiment if the field about the axis is sufficiently uniform without the transverse portions.

Provided the length of each of the air gaps 6 and 7 does not exceed about one-third (and in some cases onehalf of the diameter of the shields 8, 9 and 10, and the ampere-turns be appropriately adjusted), it is not necessary to provide outer solenoids on each side of the air gaps. Thus in Fig. 3 an arrangement is shown with a single outer solenoid 20, over the middle solenoid 2, for regulating the field in both the air gaps 6 and 7. In this embodiment the transverse portions 11 to 14 of the magnetic screens are retained to control the fringing fiux and thus assist in maintaining, off the axis, uniformity of the field across the gaps.

In the embodiment of Fig. 4, a traveling wave tube 21 is shown mounted in a unitary assembly according to the invention which comprises the several solenoids of the focussing coil arrangement, like parts of which are identified by the same reference numerals as the corresponding parts in the previously described embodiments, together with a pair of waveguide feeds, 22 and 23, and mounting arrangements (not shown in detail) for the traveling wave tube. The waveguides 22 and 23 are of rectangular section and are terminated in tuneable matching stubs 24 and 25 respectively. The traveling wave tube contains a helix, within a smaller diameter envelope portion 26 and an electron gun within a larger diameter envelope portion 27. The other end of the envelope is terminated in an electron collector electrode which is fitted with a heat radiator 28. The electron gun, helix and collector electrode are not shown in the drawing, but the limits of the axis over which the magnetic focussing field is required are indicated by the previously used reference numerals 4 and 5. The electron gun end of the tube carries a conventional type of radio receiving tube base 29 which is provided with a locating flange 30 which engages with suitable fixing means, not shown, on the end-plate 18 of the assembly. The other end of the tube is supported in the end-plate 19. The whole assembly is clamped up by means of tie-rods 31 of low reluctance material, assembled in a squirrel-cage arrangement about the assembly so as to function also as a magnetic circuit return path in the same manner as the external screens 20 of the previously described embodiments.

The inner solenoids 2 and '3 have internal diameters such that there is sufficient clearance only for the smaller diameter envelope portion 26 of the traveling wave tube. In order to accommodate the larger envelope portion 27, the inner solenoid 1 has, therefore, to be of a correspondingly larger internal diameter than that of the other inner solenoids. At the same time it is desirable to maintain the same effective number of ampere-turns per unit length throughout the inner solenoid system. The magnetic screen 15 is, therefore, effectively continued over the end of the solenoid 1 remote from waveguide 23 to project reentrantly within the solenoid, up to the position 4, thus concentrating the flux of the solenoid 1 over the region between the position 4 and the transverse magnetic screen portion 11. As a matter of practical construction, the reentrant portion 32 is formed as a sleeve attached to the end-plate 18. To provide uniformity of flux within the traveling wave tube as it passes through the waveguide 23, the central aperture in the transverse screen portion 11 is made the same as that in the screen portion 12, these transverse portions, as explained earlier, functioning as pole pieces.

It was mentioned previously that it has, in some cases, been found desirable to increase the magnetic beam constraining field towards the output end of the traveling wave tube. Applicant has found that an increase of some percent in the magnetic field strength in the region of the output waveguide 22 should be made. For this purpose it is suificient to increase the number of turns on solenoids 17 and 3 by 30 percent and 50 percent respectively, thus obtaining a monotonic increase of field across the air gap presented by the waveguide 22.

Although the invention has been particularly described with reference to magnetic systems in which it is required to maintain either a uniform flux or a monotonic increase of flux an airgap transverse to a given axis, as has already been pointed out, it can also be used to regulate the field across the airgap in other ways. Thus it would be possible thereby to nullify the flux across an airgap in a magnetic field which contained the electron gun of a velocity modulation tube, so obtaining a zero field condition at the cathode required in some types of electron flow. Alternatively, the invention readily enables a predetermined curvature of the magnetic flux lines in an air gap to be obtained with uniform, but different, fields to either side.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

A traveling wave beam control device comprising, as a unitary assembly, a plurality of inner solenoids mounted end to end, means for mounting a traveling wave tube within said solenoids along the axis thereof, a wave guide mounted between the ends of adjacent solenoids for coupling to a traveling wave tube mounted within said solenoids magnetic screens around said solenoids and extending over the adjacent ends of said inner solenoids, outer solenoids on said screens adjacent to said wave 5 guide, bars of magnetic material extending between said end plates and serving to clamp together said assembly and to provide an external low reluctance magnetic path between the ends thereof and said plates of magnetic material extending beyond the outer periphery of said outer 10 solenoids and said bars and an annular extension of magnetic material on one of said end plates extending internally of the inner surface of one of said inner solenoids.

References Cited in the file of this patent UNITED STATES PATENTS 

